Optical communication systems increasingly employ wavelength division multiplexing (WDM) techniques to transmit multiple information signals on the same fiber, and differentiate each user sub-channel by modulating a unique wavelength of light. WDM techniques are being used to meet the increasing demands for improved speed and bandwidth in optical transmission applications. Optical switches are often realized in optical waveguides that can be manufactured with low cost and enable easy multiplexing and de-multiplexing of the WDM signal using waveguide grating routers (WGR). Switching in waveguides is often accomplished by applying phase or amplitude changes using an electrooptic effect or a thermooptic effect.
Planar lightwave circuit technology permits the large-scale integration of optical functionality on a single chip enabling applications such as reconfigurable add-drop multiplexers, tunable filters and dispersion compensators. Waveguide switches are typically constructed using Mach Zehnder Interferometers (MZI) combined with thermooptic phase shifters. These thermal phase shifters suffer from high power consumption and thermal crosstalk, limiting the scale of integration. Recently, micro-electromechanical systems (MEMS) based actuators have been explored for optical switching. Electrostatically actuated MEMS devices are attractive for large scale switching applications because of their inherently low power consumption.
U.S. patent application Ser. No. 10/081,498, to Aksyuk et al., entitled “Planar Lightwave Wavelength Device Using Moveable Mirrors,” discloses optical switches that adjust the phase of an optical signal by varying the path length of the optical signal using one or more moveable mirrors. A number of optical devices incorporating moveable mirrors are disclosed. In an exemplary 2-by-2 optical switch, two waveguides configured to include a coupler region carry light signals in both directions. A mirror is positioned at the output of each waveguide. The position of at least one of the mirrors may be adjusted along the optical path and the mirrors reflect the light exiting from the end of the waveguides back into the same waveguide after an adjustable phase delay due to the round trip optical path through an adjustable air gap between the waveguides and corresponding mirrors. The position of the mirrors may be controlled, for example, using micromachined control elements, such as micro electro mechanical systems (MEMS) switches, that physically move the mirror along the light path. In one implementation of the disclosed optical switches, the MEMS mirrors move out of the plane of the wafer containing the waveguide to change the phase of the optical signal. To package the device, a first MEMS chip must be adhered in a perpendicular relationship to a second silica waveguide chip, resulting in a challenging alignment and assembly process.
U.S. patent application Ser. No. 10/387,852, to Aksyuk et al., entitled “Waveguide/MEMS Switch,” discloses an improved waveguide/MEMS switch having a waveguide device and a MEMS device that moves in the same plane as the waveguide device. The planar MEMS device includes a moveable mirror optically coupled to a waveguide of the waveguide device and adapted to move parallel to the plane of the MEMS device. Thus, in the disclosed waveguide/MEMS switch, the mirror moves in the same plane of the waveguide chip. The MEMS device may be adhered to the waveguide device in a conventional manner using a flip chip bonder, thereby simplifying the packaging and assembly of the waveguide/MEMS switch. Nonetheless, the waveguide/MEMS switch requires the fabrication of the two distinct planar waveguide and MEMS devices and the subsequent assembly into a single waveguide/MEMS switch. A need therefore exists for a monolithic waveguide/MEMS switch on a single wafer, such as a silicon-on-insulator (SOI) wafer, and a method for fabricating such monolithic waveguide/MEMS switches.